SCIENCE FILE

Rock group began with a galactic hit

Billions of years ago, a crash beyond Neptune produced the family. They're still together.

Caltech astronomers have detected a family of rocky objects in the Kuiper Belt that were formed by something hitting an object larger than Pluto.

Such groups of objects, called collisional families, are common in the asteroid belt between Mars and Jupiter, but the new family is the first to be found in the Kuiper Belt, which is beyond Neptune, more than 3 billion miles from the sun.

The parent object is a large, football-shaped rock called EL61, discovered in 2003 by Caltech astronomer Mike Brown and colleagues. The same team reported in Thursday's issue of the journal Nature that EL61 has two moons and many smaller fragments that are circling the sun in the same orbit.

EL61 is an anomaly in the belt. It is composed primarily of rock, although its surface is covered with pure ice; other objects in the belt are mostly ice. And it spins unusually fast, completing a full revolution in less than four hours.

The newly discovered objects are also ice-covered rocks, Brown said -- strong evidence that they split off from the parent.

The team concluded that the original object was spherical and about the size of Pluto until it was hit by a smaller body about 4.5 billion years ago.

"The impact made a tremendous fireball, and large icy chunks of the big object split off and went flying into space, leaving behind a huge ice-covered rock spinning end-over-end every four hours," Brown said.

It spins so fast that it has deformed into the shape of a football, he said, "but one that's a bit deflated and stepped on."

Brown predicts that EL61 will become a massive comet, about 6,000 times brighter than Hale-Bopp, which last appeared in 1997. But, he added, that won't occur for another billion years.